Ibarra calls as the biggest mistake, people jumping ships without plan or practice. The best way to make the transition is to to start by doing some of the activities of the new career even when you are in your current job, as Ibarra calls it, "test possible selves without compromising our current jobs"

Straddling is not a good business strategy. Businesses that don't focus their scant resources on specific opportunities and instead spread them across multiple projects show no strategy at all. Career change is one case where temporary straddling works. In our personal lives time is the most important scant resource and to try out our multiple "working identities" simultaneously will strain us to the hilt. But the alternative to jump first and learn to swim later is a recipe for guaranteed failure.

Even if you are not thinking of a big change, start with Ibarra's book.